As recording materials for heat-sensitive recording methods, leuco color formation type heat-sensitive recording materials have been generally employed. However, these heat-sensitive recording materials involve disadvantages, in that undesired color formation can occur in inappropriate portions due to rough handling after recording, due to heating or adherence of solvents thereto, etc., to thereby contaminate recorded images. As heat-sensitive recording materials free from these disadvantages, diazo color formation type heat-sensitive recording materials have been extensively investigated in recent years. These techniques are disclosed, for example, in Japanese Patent Application (OPI) No. 123086/82 (the term "OPI" as used herein refers to a "published unexamined Japanese patent application"), Journal of the Image Electronics Association, Vol. 11, p. 290 (1982), etc., in which images are thermally recorded on recording materials comprising diazo compounds, coupling components and basic components (including substances capable of becoming basic when heated) and then exposed to light to thereby decompose the unreacted diazo compounds, resulting in discontinuation of the color formation. According to this method, prevention of color formation (hereafter referred to as fixing) in portions where recording is unnecessary can indeed be achieved. However, even with such recording materials, precoupling sometimes gradually proceeds during storage to cause undesired coloration (fogging). For this reason, any one of color forming components is made present in the form of discontinuous particles (solid dispersion) to prevent contact between the components, whereby the precoupling is prevented. However, such involves disadvantages that storability of recording materials (hereafter referred to as shelf storage) is unsatisfactory and further color formation upon heating is deteriorated. As another countermeasure, it is known that diazo compounds are separated from coupling components into separate layers in order to minimize the contact between the components (described, for example, in Japanese Patent Application (OPI) No. 123086/82, supra). According to this method, shelf storage is improved but reduction in color formation upon heating is serious, so that the system fails to respond to high speed recording having a short pulse width, and is therefore not practical. In addition, as a method for providing satisfactory shelf storage and color formation upon heating simultaneously, it is known that either coupling component or basic substance is encapsulated by non-polar waxy substances (Japanese Patent Application (OPI) Nos. 44141/82 and 142636/82) or by hydrophobic high molecular weight substances (Japanese Patent Application (OPI) No. 192944/82). However, these encapsulation methods comprise dissolving waxes or polymeric substances in solvents thereof and dissolving or dispersing color forming components in these solvents to form capsules, which is different from the concept of conventional capsules which comprise a core substance covered with a shell therearound. For this reason, in the case that the color forming components are dissolved to form such capsules, the color forming components do not constitute the core substances of the capsules, but rather are homogeneously mixed with the encapsulating materials, whereby precoupling gradually proceeds at the interface of the capsule wall during storage so that shelf storage is not sufficiently satisfactory. Furthermore, when the color forming components are dispersed to form capsules, no color formation occurs unless the capsule walls are melted by heating; this results in decreased color formation upon heating. Furthermore, a problem is encountered in production in that solvents used for dissolving the waxes or the polymeric substances should be removed after the capsules were formed, but such cannot be done sufficiently satisfactory.
Therefore, for solving the foregoing problems, a heat-sensitive recording material has been disclosed, made by a method of microencapsulation which comprises incorporating at least one of the components participating in the color forming reaction in a core substance and forming a wall around the core material by polymerization (Japanese Patent Application (OPI) No. 190886/84, corresponding to U.S. Pat. Ser. No. 600,267).
Even in the heat-sensitive recording material according to this microencapsulation method, however, it was impossible to eliminate a disadvantage that the background areas (areas excluding recorded images) are stained to a somewhat yellow color when photofixation follows thermal recording.